We propose to study the properties of isolated microtubule initiation sites from mouse neuroblastoma and mouse fibroblast cells. The sites will be isolated and the polarity of growth in vitro determined from the rate of microtubule elongation by darkfield light microscopy. We plan, in addition, to purify the nucleation sites and determine whether they contain tubulin. Studies are also proposed on determining the number of tubulin and actin genes in vertebrates. Using our newly isolated cDNA clones from chicken brain mRNA for tubulin and actin, we will search for genome clones to determine the number of different copies. Further experiments are proposed for looking at selective gene expression. Finally, experiments are proposed for looking at post-translational modification of tubulin and microtubule associated proteins.